The failure of the Central Nervous System (CNS) axons to regenerate has been attributed primarily to the presence of inhibitory components within CNS environment that bind receptors on axons to stimulate inhibitory signaling cascades. Given that elevation of cyclic AMP (cAMP), a ubiquitous intracellular second messenger, enhances nerve regeneration and overcomes the inhibition of neurite outgrowth, molecules that regulate cAMP in neurons are prime candidates for molecular intervention in the context of nervous system repair. Indeed, rolipram, an inhibitor of type four phosphodiesterases (PDE4s), the enzymes responsible for cAMP degradation, has been shown to overcome the inhibition of neurite outgrowth (28). However, rolipram is a general inhibitor of all PDE4s (PDE4A-D) and causes many side effects in patients that preclude its use in the clinic. To treat patients effectively, a more specific inhibitor with fewer side effects is required. To date, the specific PDE4 isoforms that are targets for rolipram in the disinhibition of neurite outgrowth remain to be determined. Our major hypothesis is that phosphodiesterase 4A (PDE4A) plays a major role in the regulation of cAMP after nerve injury and is an attractive target for therapeutic intervention in neurodegenerative diseases, such as spinal cord injury. Our preliminary data indicate that: 1. Expression of the p75 neurotrophin receptor (p75NTR), a receptor implicated in the inhibition of neurite outgrowth and nerve regeneration, leads to decreased cAMP levels in neurons. 2. Inhibition of PDE4s with rolipram rescues the p75NTR induced decrease in cAMP. 3. PDE4A5 forms a complex with p75NTR and targets cAMP degradation to the membrane. 4. The extreme C-terminus of PDE4A5 directly interacts with the intracellular domain of p75NTR. In the research proposed in this fellowship application, we will determine the role of PDE4A in regulating the inhibition of neurite outgrowth (Specific Aim 1). We will examine the role of PDE4A in the regulation of both cAMP and Rho signaling, two intersecting pathways known to play major roles in the inhibition of neurite outgrowth (Specific Aim 2). We will extend these studies by examining the role of PDE4A in axonal regeneration in vivo (Specific Aim 3). There are an estimated 250,000 individuals with permanent disabilities resulting from spinal cord injuries living in the United States with 11,000 new injuries reported every year. At present, there are no effective pharmacologic agents that promote complete functional recovery after injury. The identification of novel, specific drug targets, such as PDE4A, could greatly enhance our ability to overcome the inhibition of axonal regeneration that proves so costly for victims of spinal cord injury. [unreadable] [unreadable]